The present invention relates to a resonator assembly comprising a helix resonator consisting of a conductor wound as a cylindrical coil, and a housing at least partly made of conductive material and enveloping the helix resonator.
The present invention relates primarily to resonators used in filters of mobile telephone systems, although the invention can also be utilized in other contexts. Hereinafter, however, the invention will be described by way of example particularly with reference to mobile telephone systems.
A straight quarter wave resonator is previously known comprised of a straight conductor inserted into a metal housing. Such a resonator is suited for use in filters of mobile telephone system, for example. In the GSM system (Global System for Mobile communications), for example, wherein the frequencies used are approximately 900 Mhz, such a quarter wave coaxial resonator is about 80 mm long. As components become smaller and smaller, in practice, a resonator of the size described above has turned out to be too big. A solution for decreasing the resonator length has been to equip the resonator with a xe2x80x9ccapxe2x80x9d enabling the length of the straight part of the resonator to be decreased, based on the fact that the cap loads the resonator by lowering the frequency back to the 900 Mhz area. High capacitance between the cap and the resonator cavity, however, causes the quality factor, i.e. Q factor, to drop sharply. Sometimes a resonator assembly of this kind thus fails to produce high quality factors required by the resonators of base station filters, for example.
Furthermore, a resonator called a helix resonator is also previously known comprising a quarter wave long resonator wound as a cylindrical coil. The resonator is then comprised of a conductor wound to form a coil of cylindrical form using thread or section wire. The helix resonator is inserted into a housing of conductive material serving as a ground plane, one end of the resonator being coupled thereto. The other end of the helix resonator is left open. The helix resonator is significantly smaller than a coaxial resonator with corresponding characteristics. However, the helix resonator suffers from the same problem, i.e. too low a quality factor, as the above-described straight coaxial resonator equipped with a cap.
An object the present invention is to solve the problem described above and provide a resonator assembly having a high quality factor and taking up a relatively small space. This aim is achieved by a resonator assembly of the invention comprising a helix resonator consisting of a conductor wound as a cylindrical coil, and characterized by the conductor that forms said cylinrical coil continuing as a straight conductor part, which extends from the cylindrical coil substantially in the direction of the longitudinal axis of the cylindrial coil, whereby said straight conductor part constitutes a coaxial resonator, and said housing enveloping the resonator formed by a combination of the helix resonator and the coaxial resonator.
The invention is based on the idea that combining a helix resonator and a straight coaxial resonator into one resonator provides a resonator assembly having significantly better characteristics than the known resonators. In the resonator assembly of the invention, the conductor forming the coil of the helix resonator can continue as a straight conductor part constituting a coaxial resonator, in other words the conductor can be the same physical wire whose first end is shaped as a spiral and whose second end is shaped as a straight conductor part. Alternatively, the conductors can be two separate wires coupled with each other in a manner known per se, for example by soldering. Hence, one wire is shaped as a spiral and the other as a straight conductor part.
Practical experiments show that a resonator assembly comprised of a helix resonator and a straight coaxial resonator significantly enables space to be saved since the total length of the resonator assembly can thus be decreased, compared with the known straight coaxial resonator, without the quality factor, i.e. Q factor, of the resonator consequently being significantly lowered. Hence, the most significant advantage of the resonator assembly of the invention is that it is space-efficient without the resonator quality factor consequently being lowered. The resonator assembly of the invention is suited for use in RF filters, for example.
In a preferred embodiment of the resonator assembly of the invention the diameter of the conductor forming the helix resonator is different from the diameter of the conductor part forming the coaxial resonator. This embodiment of the invention enables an impedance change to take place at the joining point of the conductors where the diameter changes. Hence, a necessary impedance change in the assembly used can be provided by means of dimensioning.